Heat exchangers/catalysts are often a channel system having a body, which is formed with a large number of juxtaposed small channels through which flows a fluid or fluid mixture, which, for example, is to be converted. The channel systems are made of different materials, such as ceramic materials or metals, for example stainless steel or aluminium.
Channel systems made of ceramic materials has a channel cross-section, which usually is rectangular or polygonal, for example hexagonal. The channel system is made by extrusion, which means that the cross-section of the channels is the same along the entire length of the channel, and the channel walls will be smooth and even.
In the manufacture of channel bodies of metals, a corrugated strip and a flat strip are usually wound about a spool. This results in channel cross-sections, which are triangular or trapezoidal. Most channel systems of metals that are available on the market have channels of the same cross-section along their entire length and have, like ceramic channel bodies, smooth and even channel walls. Both these types may be coated with a coating, for example in a catalyst with a catalytically active material.
What is most important in the context is the heat, moisture and/or mass transfer between the fluid or fluid mixture flowing through the channels and the channel walls in the channel system.
In channel systems of the above type, used with for example internal combustion engines in vehicles or in the industry, with relatively small cross-sections of the channels and fluid velocities commonly used in these contexts, the fluid flows in relatively regular layers along the channels. The flow thus is essentially laminar. Only along a short distance at the inlet of the channels, a certain flow occurs transversely to the channel walls.
As is generally known in the art, a boundary layer is formed in laminar fluid flow next to the channel walls, where the velocity is essentially zero. This boundary layer significantly reduces the mass transfer coefficient, above all in the case of what is referred as fully developed flow, in which the heat, moisture and/or mass transfer occurs mainly by diffusion, which is relatively slow. The mass transfer coefficient is a measure of the mass transfer rate and should be great so as to obtain high efficiency of the heat exchange and/or the catalytic conversion. To increase the mass transfer coefficient, the fluid must be made to flow toward the surface of the channel side so that the boundary layers are reduced and the flow transfer from one layer to another is increased. This may take place by what is referred to as turbulent flow. In smooth and even channels, the laminar flow turns turbulent when the Reynolds number reaches values above approx. 2000. If one wishes to reach Reynolds number of this magnitude in the channels in the channel systems that are here involved, considerably higher fluid velocities are required than is normal in these contexts. At the low Reynolds numbers that are involved in channel systems of the type described above, it is therefore necessary to create turbulence by artificial means, such as by arranging special flow directors in the channels.
U.S. Pat. No. 4,152,302 discloses a catalyst with channels, in which flow directors are arranged in the form of transverse metal flaps punched from the strip. A catalyst with flow directors significantly increases the heat, moisture and/or mass transfer. However, at the same time also the pressure drop increases dramatically. The effects of the pressure drop increase have, however, been found to be greater than the effects of the increased heat, moisture and/or mass transfer. The pressure drop depends on, inter alia, the configuration, dimension and geometry of the flow directors. However, it is generally known that said types of flow director produce an excessive pressure drop, and therefore they have not been used commercially to any considerable extent.
EP 0 869 844 discloses turbulence generators extending transversely to the ducts of the catalyst or heat/moisture exchanger to obtain an improved ratio of pressure drop to heat, moisture and/or mass transfer.
WO 2007/078240 discloses flow converters extending transversely to the channels. However, further improvements of the ratio of pressure drop to heat, moisture and/or mass transfer is always a desire for the manufacturer of such systems.